First off: I would warn against basing any decision like this on month to month balance considerations. Things change so often in WoW that you should really decide based on how much you enjoy a particular classes playstyle, toolbox, and yes, even aesthetics. Healing Balance is Pretty Good right now. All five healing specs have a role to play, and a (25 man) raid which doesn't have the flexibility to see them all represented is a 25 man raid that is going to run into trouble. At the five man level, all five specs are more than capable of handling any content you'll see, though Discipline Priests in particular may find Heroics somewhat more challenging than most due to increased gear dependency.
That said, the current state of PvE healer balance at the high end is this: Priests are somewhat above average at Group and Raid healing - this is reflected by the nerfs to Prayer of Healing in the upcoming 4.0.6 patch, but those nerfs don't really change the overall state of things. Additionally, Priests bring extremely powerful cooldowns to a group such as Pain Suppression, Guardian Spirit, Leap of Faith, Divine Hymn, and Power Word: Barrier, all of which are defining abilities that have had entire strategies built around them. If you see raids specifically looking for Priests to heal, this is why.
Shaman by contrast, are currently the kings of PvP healing because they are extremely durable, mobile thanks to the new Spiritwalker's Grace ability, and bring substantially more offensive utility than other healers thanks to Heroism/Bloodlust, Wind Shear, Totems and Purge. In PvE, every 25 man raid wants at least one Resto Shaman for Mana Tide - and 10 mans love to have it as well. Mana Tide is the single most powerful mana regen ability in the game, and it is absolutely essential for success for many groups in many encounters. Additionally, you may find it easier to gear up a Shaman for healing because there will be less competition on mail pieces with intellect than you will find on similar cloth pieces - and because if you already have decent DPS gear, you always have the option of DPSing your way to a healing set.
My advice to you? Try out healing on the shaman. Chain Heal, Spiritwalkers Grace, Riptide and Healing Rain are all enormously fun abilities, and the Shaman healing style is unique among healers in WoW. Unless you're pushing bleeding edge content the class balance issues seen there are going to be largely moot for you - what matters much more is your own competence and gear.
As an aside, checking the WoW forums will always find you nothing but gripes. A saying I'm fond of is that the forums are as much an indicator of players happiness as a hospital is an indicator of public health.
Fairly subjective question, but I'll do my best for tips.
NB: I've not played demo warlock at 85, just hunter, shaman and druid - I've played warlock at lower levels in PvP though, and I've got general PvP experience. If a warlock comes along, I'll happily upvote his answer in favour of mine - but in lieu of that happening, I'm going to contribute what I can.
As a demo warlock, you want to be getting the most out of your demon at all times. You also want to choose the right demon for the right situation. For solo'ing/questing, I'd take a felguard/felhound for the survivability bonuses, but for PvP despite felguard being so popular I tend to favour succubus since it has CC abilities that can allow you to healthstone/potion/whatever after that first attack.
The other thing to do is remember that your cooldowns exist, such as demon form - you want to actually use that, since it provides a massive bonus for a short period of time. A lot of people end up not using it because they "don't want to waste it", and therefore spend an entire match saving their cooldown for "emergencies" that never occur. Don't be that person. Cooldowns aren't that long any more, and as long as you're careful when you use it, you shouldn't ever "waste" it. You should definitely use it if an objective is at stake (eg a flag or a base).
A general bit of (pug) PvP advice: try to balance sticking with your team-mates with going after objectives - too much sticking with your team and you probably wind up grinding honor in the middle of the map (because, you know, teams tend to be pretty noobish in PuG PvP), but on the other hand too much going after objectives can leave you fruitlessly throwing corpses at the enemy flag/base/whatever. Without being rude to your team-mates, gentle persuasion towards objectives can sometimes help when people are mindlessly grinding honor, but there are no guarantees. It's sometimes helpful to remind people that quick wins are worth more honor than long, painfully-slow honor-grind matches.
In terms of World PvP, since you are a cloth-wearing class the best advice I can give you is to buy/make stealth-detection potions, since your main enemy will be rogues who can sneak up and do a lot of damage very quickly before you have a chance to respond. Keeping your distance from other melee classes is also important, but rogues are the trickiest in this aspect (warrior's charge notwithstanding).
World PvP is a bit difficult to provide tips for, since more often than not it's simply a case of "outnumber, outgun or outmanoeuvre". The latter meaning "flee! run for your life!". I've not come across any effective "anti-ganking" techniques, other than running towards friendly civilization, getting a friendly player to 'bodyguard' you or having better gear/being higher level. The problem you refer to where you get ganked while you're fighting a mob and don't realise some of the damage is being done by a player, the key to avoiding that situation is simply to be vigilant at all times when in a PvP-enabled zone. Keep looking around for enemy players and don't take anything for granted.
Hope this helps :)
Best Answer
Blizzard has done a fair job at balancing the two to make them both manageable in instances. The fundamental difference between disc and holy is the play-style in my experience. Disc is primarily about reacting prior to damage using abilities such as Power Word: Shield and Divine Aegis to mitigate damage before it happens. As a Holy spec, the healer must be aware of the damage that is about to come, and rather than being able to pre-heal the player receiving the damage, they must be able to respond to the damage in a timely manner.
In many boss battles, you’ll know when the tank is about to get beat on with a special ability. This would be the time that the Disc priest would bubble the tank and prepare to also add heals as necessary once the Shield has expired. A Holy priest would begin casting a heal early so that it finishes about the time the damage is done to the tank and managing the Chakra spell to react to whichever situation is most appropriate (group heals or focused-tank heals).
Each spec has their own damage reduction/healing cooldowns. Disc has Power Word: Barrier which lowers the damage taken for everyone in the bubble, and Pain Suppression which lowers damage for the character it is cast on. Holy has Guardian Spirit for increased healing on the target, Spirit of Redemption for 15 secs of healing if you die, and Lightwell which other players click on to return health.
Both priests must manage their cooldowns effectively to keep the tank and rest of the group alive. I’ve healed as both specs in group settings and they are both viable options for 5-mans. It all comes down to what you prefer: bubbles to help take some damage and heals to recover from it or managing your chakras and timing the spells to respond when they are needed.